Exhaust gas from internal combustion engines, power plants, industrial furnaces, heaters, diesel engines, and other devices contain harmful pollutants such as nitrogen oxides, carbon monoxide, unburned hydrocarbons and particulate matter. Exhaust gases from these sources also contain excess levels of oxygen, water vapor, and sulfur dioxide. Often times, exhaust with excess oxygen levels is known as lean exhaust and engines that generate such exhaust are referred to as lean burn engines. Oxygen levels in exhaust gases from lean burn engines can vary anywhere from 1% to 20%.
Emissions of nitrogen oxides, carbon monoxide, and hydrocarbons are subject to limits due to environmental regulations. Emission control systems containing catalysts are used to decrease the pollution caused by exhaust. The temperature of the exhaust gases from lean burn engines where emission control catalysts are located can be anywhere from 100° C. to 700° C.
A common catalyst used in such applications for the purpose of removing carbon monoxide and unburnt hydrocarbons is known as an oxidation catalyst. Prior art oxidation catalysts are made using platinum in the range of greater than 10 grams per cubic feet of the catalyst.
With the ever stricter standards for acceptable emissions, the demand on platinum group metals continues to increase due to their efficiency in removing pollutants from exhaust. Platinum group metals include platinum, palladium, ruthenium, iridium, osmium and rhodium. However, this demand along with other demands for platinum group metals strain the supply of platinum group metals, which in turn drives up the cost of platinum metal and therefore catalysts and associated emission control systems as well. In oxidation catalysts used for treating lean burn exhausts, the amount of precious metal (platinum) used constitutes about 60 to 90% of the total cost of the oxidation catalyst itself.
While palladium, which has good oxidation properties and is not as expensive as platinum, can be used instead, large amounts of palladium are typically required in an oxidation catalyst, which in turn increases the cost of the oxidation catalyst and defeats the purpose of using a lower cost platinum group metal. Prior art oxidation catalysts are made using palladium in the range of several hundred grams of palladium per cubic feet of the oxidation catalyst.
In light of the above, there is a strong need for efficient catalyst formulations that contain very low amounts of precious metal content, especially palladium or platinum, or do not require the use of platinum group metals at all.